Variable output constant pressure pump



July 26, 1966 R. L. ALLEN 3,262,393

VARIABLE OUTPUT CONSTANT PRESSURE PUMP Filed March 4, 1963 A1111 u ll INVENTOR. Robert L. Allen AT TORNEYS United States Patent 3,262,393 VARIABLE OUTPUT CONSTANT PRESSURE PUMP Robert L. Allen, Atlanta, Ga.; Carmen S. Allen, executrix of said Robert L. Allen, deceased, assignor to Georgia Tech Research Institute, Atlanta, Ga., a corporation of Georgia Filed Mar. 4, 1963, Ser. No. 262,484 11 Claims. (Cl. 103-37) This invention relates to pumps, and more particularly to a reciprocating pump which discharges varying quantitles of liquid at a constant pressure.

Reciprocating pumps are characterized by pressure pulsations resulting from the action of the piston on the liquid being pumped. These pressure pulsations are usually objectionable and become hazardous to the pump and the hydraulic system with which the pump is being used, if they become excessive.

Typical of those circumstances in which these pressure pulsations become excessive is the restricting or stopping of the discharge of an operating pump. The action of the piston on the enclosed or partially enclosed liquid will cause excessive pressure pulsations to be developed, and damage to the pump or hydraulic system is difficult to avoid.

However, there are many pump applications in which it is necessary to restrict the liquid discharge of an operating pump. These are generally pump applications in which it is required that the liquid discharge of the pump vary in accordance with the varying demands of the hydraulic system. These applications generally require not only that excessive pressure pulsations not develop as the liquid discharge of the pump decreases but also that pressure not decrease as the liquid discharge increases.

The improved reciprocating pump disclosed herein fully meets both of these requirements. It is a reciprocating pump which discharges at a constant pressure any quantity of liquid from zero to the maximum capacity of the pump in response to the demands for liquid placed upon the pump. The rate of liquid discharge is freely variable within the range established by the maximum capacity of the pump, and excessive pressure pulsations when the rate of liquid discharge is reduced and losses of pressure when the rate of liquid discharge is increased are completely eliminated. Moreover, the invention tends to absorb the normal, but objectionable, pressure pulsations which have always characterized reciprocating pumps, even when the discharge rate is not being varied.

The foregoing improvements in reciprocating pump performance are obtained by an improved pump having a pumping chamber which expands to accommodate liquid not being discharged or which would otherwise cause excessive pressure in the pump or hydraulic system if restricted to a specific volume and exposed to the action of the piston. The degree of expansion of the pumping chamber is inversely related to the rate of liquid discharge from the pump, but is never so great as to result in the loss of pressure.

These and other features of the invention will be more clearly understood from the following detailed description and the accompanying drawings in which like characters of reference designate corresponding parts in all figures, and in which:

FIGURE 1 is a partial section view of the pump taken in a vertical plane containing the centerline of the pump and showing the housing and caps in section and the sleeves, piston, and other elements of the pump in elevation.

FIGURE 2 is a partial section view of the pump taken in a vertical plane containing the centerline of the pump and showing the housing, caps, and sleeves in section 3,262,393 Patented July 26, 1966 'ice and the piston and other elements of the pump in elevation.

FIGURE 3 is a partial section view of the pump taken in a vertical plane containing the centerline of the pump and showing the housing, caps, sleeve-s, packing, and piston in section and other elements of the pump in elevation.

These figures and the following detailed description disclose a preferred specific embodiment of the invention, but the invention is not limited to the details disclosed since it may be embodied in other equivalent forms.

This invention is best understood by considering it to be a reciprocating pump having a pumping chamber which varies in configuration and size as the discharge rate of the pump varies. In the specific embodiment of the invention described herein, there are two pumping chambers. The pumping chambers are in a housing 10 which has a cylindrical bore 11 extending through it. This bore 11 is closed at one end by a cap 12 and at its other end by a cap 13.

Two cylindrical sleeves, 14 and 15, are slidably positioned within the bore 11. One sleeve 14 is adjacent to the cap 12 and the other sleeve 15 is adjacent to the cap 13. The hole 16 through sleeve 14 and the hole 17 through sleeve 15 are cylindrical and the centerlines of the holes 16 and 17 coincide with the centerline of the bore 11.

A piston 18 extends from the sleeve 14 to the sleeve 15 and has one end 19 extending into the hole 16 through the sleeve 14 and its other end 20 extending into the hole 17 through the sleeve 15. The piston 18 is cylindrical in shape and the centerline of the piston 18 coincides with the centerlines of the bore 11, the hole 16, and the hole 17. The end 19 of the piston 18 closes the hole 16 in the sleeve 14 and prevents the passage of a liquid through the hole 16. Similarly, the end 20 of piston 18 closes the hole 17 in the sleeve 15 and prevents the passage of a liquid through the hole 17. However, the outside diameter of the piston 18 is sufiiciently less that the diameters of the holes 16 and 17 to permit the piston 18 to be slidably movable within the holes 16 and 17.

Although the sleeves 14 and 15 are slidable within the bore 11 of the housing 10, the outside diameters of the sleeves 14 and 15 are large enough to result in the forming of two substantially liquid-tight barriers in the bore 11. One barrier is formed by the sleeve 14 and the end 19 of the piston 18 within it and the other barrier is formed by the sleeve 15 and the end 20 of the piston 18 within it.

Five springs, 21, extend from the sleeve 14 to the sleeve 15. Each spring 21 has one end inserted into a recess 22 in the sleeve 14 and its other end inserted into a recess 23 in the sleeve 15, and all five springs 21 extend parallel to the centerline of the piston 18 within the bore 11. The springs 21 are equidistantly spaced around the piston 18 and uniformly urge the sleeves 14 and 15 apart.

As a result of the action of the springs 21, the face 24 of the sleeve 14 is forced toward and against the cap 12 and the face 25 of the sleeve 15 is forced toward and against the cap 13. The length of the piston 18 is less than the distance between the caps 12 and 13 and when the end 19 of the piston 18 is engaging the cap 12, an initial pumping chamber 26 is formed in the hole 17 through the sleeve 15. When the end 20 of the piston 18 is engaging the cap 13, a second initial pumping chamber 27 is formed in the hole 16 through the sleeve 14.

It is into an out of these initial pumping chambers 26 and 27 that liquid moves when the pump is operating. The liquid passes into the pump through an inlet hole 28 in the housing 10 and accumulates in a reservoir space 29 surrounding the piston 18 between the sleeves 14 and 15. A duct 30 extends through the piston 18 from this reservoir space 29 to the end 19 of the piston 18 and a similar duct 31 extends through the piston 18 from reservoir space 29 to the end of the piston 18. At end 19 of the piston 18 is a suction valve 31 positioned over the end 32 of the duct and at end 20 of the piston 18 is a suction valve 33 positioned over the end 34 of the duct 31. The suction valve 31 permits liquid to pass from the reservoir space 29 through the duct 30 and into the initial pumping chamber 27 and the suction valve 33 permits liquid to pass from the reservoir space 29 through the duct 31 and into the initial pumping chamber 26. The suction valves 31 and 33 prevent all flow of liquid from either the initial pumping chamber 26 or the initial pumping chamber 27 into the reservoir space 29.

The liquid is discharged from the pump through a passage 35 in the cap 12 and through a passage 36 in the cap 13. The passage 35 has a check valve 37 positioned within it and the passage 36 has a check valve 38 positioned within it. These check valves 37 and 38 are of known type and the check valve 37 is placed within the passage 35 in the known manner by making the cap 12 of two parts, 12a and 12b. Similarly, the check valve 38 is placed in the passage 36 in known manner by making the cap 13 of two parts, 13a and 13b. The check valve 37 is arranged to permit liquid under greater pressure than that exerted by its check spring 39 to flow through the passage 35 from the initial pumping chamber 27. Similarly, the check valve 38 is arranged to permit liquid under greater pressure than that exerted by its check spring 40 to flow through the passage 36 from the initial pumping chamber 26. The check valve 37 will prevent all flow of liquid into the pump through the passage 35 and the check valve 38 will prevent the flow of all liquid into the pump through the passage 36.

Piston 18 motion in the hole 16 through the sleeve 14 and in the hole 17 through the sleeve 15 is obtained by inserting a crank shaft 41 with a nub 42 on its inserted end through an aperture 43 in the housing 10. The nub 42 is eccentrically placed with respect to the centerline of the crank shaft 41 and as the crank shaft 41 rotates, the nub 42 will follow a circular path. This circular path of the nub 42 has an axis of motion parallel to the centerline of the piston 18 and an axis of motion perpendicular to the centerline of the piston 18.

The nub 42 is inserted into and movable within a slot 44 in a bushing 48 in the piston 18. This slot 44 has sufiicient length perpendicular to the centerline of the piston 18 to accommodate the motion of the nub 42 along its circular path with respect to that axis of motion perpendicular to the centerline of the piston 18. However, the width of the slot 44 parallel to the centerline of the piston 18 will not accommodate the motion of the nub 42 along its circular path with respect to that axis of motion parallel to the centerline of the piston 18. As a result, the rotation of the crank shaft 41 and the resulting circular path of the nub 42 will cause a reciprocating motion of the piston 18 along a line of motion coinciding with the centerline of the piston 18. This motion of the piston 18 alternately drives the end 19 of the piston 18 toward the cap 12 and the end 20 of the piston 18 toward the cap 13. The aperture 43 through which the crank shaft 41 enters the housing 10 is sealed by packing of known type which permits the crank shaft 41 to freely rotate, but which prevents the escape of liquid from the reservoir space 29 through the aperture 43.

When the pump is operating, the crank shaft 41 rotates and liquid enters the pump through the inlet hole 28. As the end 19 of the piston 18 moves away from the cap 12, a vacuum is created in the initial pumping chamber 27 which draws liquid from the reservoir space 29 through the duct 30 and the suction valve 37 and into the initial pumping chamber 27. The continuing rotation of the crank shaft 41 causes the piston 18 to reverse its motion and the end 19 of the piston 18 to be driven 4 toward the cap 12. This causes the end 19 of the piston 13 to exert pressure on the liquid in the initial pumping chamber 27 and liquid to be drawn into the initial pumping chamber 26 where pressure will be exerted upon it by the end 20 of the piston 18 with the continuing rotation of the crank shaft 41.

The pressure alternately exerted by the end 19 of the piston 18 on the liquid in the initial pumping chamber 27 and by the end 21) of the piston 18 on the liquid in the initial pumping chamber 26 will alternately tend to force the liquid in the initial pumping chamber 27 through the passage 36 in the cap 13 and the liquid in the initial pumping chamber 27 through the passage 35 in the cap 12. However, if the pressure exerted by the piston 18 on the liquid in the initial pumping chambers 26 and 27 exceeds the pressure exerted by the springs 21 on the sleeves 14 and 15, the pressure of the end 19 of the piston 18 on the liquid in the initial pumping chamber 27 will cause liquid to be forced between the cap 12 and the face 24 of the sleeve 14 and the pressure of the end 28 of the piston 18 on the liquid in the initial pumping chamber 26 will cause liquid to be forced between the face 25 of the sleeve 15 and the cap 13. When this occurs, the sleeve 14 will be forced away from the cap 12 against the pressure of springs 21 and the sleeve 15 will be forced away from the cap 13 against the pressure of springs 21.

Thus, when pressure is exerted on liquid in the initial pumping chamber 27 by the end 19 of the piston 18, the liquid will be either ejected through the passage 35 through the cap 12 or forced into an auxiliary pumping chamber 47 formed between the cap 12 and the face 24 of the leeve 14. Similarly, when pressure is exerted on liquid in the initial pumping chamber 26 by the end 20 of the piston 18, the liquid will be either ejected through the passage 36 through the cap 13 or forced into an auxiliary pumping chamber 46 formed between the cap 13 and the face 25 of the sleeve 15.

Liquid in the initial pumping chamber 27 will not be forced into, the auxiliary pumping chamber 47 unless the pressure exerted by the end 19 of the piston 18 on the liquid in the initial pumping chamber 27 exceeds the pressure exerted by the springs 21 on the sleeve 14. Similarly, liquid in the initial pumping chamber 26 will not be forced into the auxiliary pumping chamber 46 unless the pressure exerted by the end 28 of the piston 18 on the liquid in the initial pumping chamber 26 exceeds the pressure exerted by the springs 21 on the sleeve 15. Liquid enters the auxiliary pumping chambers 46 and 47 only if the pressure in the initial pumping chambers 26 and 27 exceeds that pressure selected for the pressure exerted by the springs 21.

The size, speed, and stroke of the piston 18 and the sizes of the passages 35 and 36 are selected in the known manner to provide that constant pressure and that maximum rate of liquid discharge desired. The springs 21 are selected to exert a pressure on the sleeve 14 and 15 equal to the constant pressure desired.

If liquid flows freely through the passage 35 through the cap 12 and the passage 36 through the cap 13, the auxiliary pumping chambers 46 and 47 will not form and liquid will be discharged from the initial pumping chambers 26 and 27 at the constant pressure and the maximum rate of discharge. However, if the discharge of the liquid through the passages 35 and 36 is restricted or pressure in exces of the constant pressure is for any other reason exerted on the liquid in the initial pumping chambers 26 and 27, the auxiliary pumping chambers 46 and 47 will be formed and will be sufiiciently large to accommodate liquid which the rate of liquid discharge will not accommodate without the development of pressure in excess of the constant pressure. The fact that the sleeves 14 and 15 will yield to provide an auxiliary pumping chamber 46 or 47 whenever the constant pressure is exceeded within the pump insures that excessive pressures resulting from restricting or stopping the discharge of liquid or from any other cause, will not be developed within the pump or the hydraulic system with which the pump is associated.

When the pressure in the pump drops to or below the constant pressure as the end 20 of the piston 18 moves away from the cap 13 after a discharge stroke or as the cause for the excessive pressure in the pump disappears, the springs 21 force the sleeve 15 toward the cap 13 and any liquid in the auxiliary pumping chamber 46 will be returned to the initial pumping chamber 26 where it will be discharged as it returns or stored until the next discharge stroke. The springs 21 produce the same result with respect to the auxiliary pumping chamber 47 and the initial pumping chamber 27.

Any liquid stored in the initial pumping chamber 26 is supplemented by additional liquid through the suction valve 33 and any liquid stored in the initial pumping chamber 27 is supplemented by additional liquid through the suction valve 31. Thus, at the beginning of each discharge stroke of the pump, the initial pumping chambers 26 and 27 are completely full of liquid and the auxiliary pumping chambers 46 and 47 are completely empty.

The auxiliary pumping chambers 46 and 47 provide space for all or any part of the liquid in the initial pumping chambers 26 and 27 at the beginning of a discharge stroke of the pump. However, since they provide space only for the quantity of liquid necessary to prevent pressures in excess of the constant pressure being developed in the pump, the auxiliary pumping chambers 46 and 47 can not cause loss of pressure. Liquid needed in the initial pumping chambers 26 and 27 to maintain the constant pressure does not enter the auxiliary pumping chambers 46 and 47 or is forced from the auxiliary pumping chambers 46 and 47 by the action of springs 21.

It will be obvious to those skilled in the art that many variations may be made in the embodiments here chosen for the purpose of illustrating the present invention without departing from the scope thereof as defined by the appended claims.

What is claimed as invention is:

1. A pump for discharging varying quantities of liquid at a constant pressure, said pump comprising, in combination, a housing having a first end, a second end, a cylindrical bore extending through it from its first end to its second end and with a centerline, an inlet hole extending through it from the bore and positioned along the length of the bore between the first end and the second end of the housing, and an aperture extending through it from the bore and positioned between the first end and the second end of the housing; a first cap positioned across and closing the bore at the first end of the housing, said cap having a first passage extending through it from the bore; a second cap positioned across and closing the bore at the second end of the housing, said cap having a second passage extending through it from the bore; a first sleeve slidably positioned in the bore between the first cap and the position of the inlet hole along the length of the bore and having a hole extending through it with a centerline which coincides with the centerline of the bore; a second sleeve slidably positioned within the bore between the second cap and the position of the inlet hole along the length of the bore and having a hole extending through it with a centerline which coincides with the centerline of the bore; a piston having a cylindrical exterior surface, a first end slidably movable in the hole extending through the first sleeve, -a second end slidably movable in the hole extending through the second sleeve, a centerline coinciding with the centerline of the bore, a first duct extending through it from its first end to a portion of its exterior surface between the first sleeve and the second sleeve, a second duct extending through it from its second end to a portion of its exterior surface between the first sleeve and the second sleeve, a slot in its exterior surface with its length perpendicular to the centerline of the piston, and a length less than the distance between the first cap and the second cap; a plurality of springs extending between the first sleeve and the second sleeve, equidistantly positioned around the piston, and urging the first sleeve toward the first cap and the second sleeve toward the second cap with a force substantially equal to the constant pressure; a first suction valve located at the first end of the piston and positioned to permit the flow of liquid only from that end of the first duct at the first end of the piston; a second suction valve located at the second end of the piston and positioned to permit the flow of liquid only from that end of the second duct at the second end of the piston; a first check valve located in the first passage and positioned to permit the flow of liquid only from the bore; a second check valve located in the second passage and positioned to permit the flow of liquid only from the bore; a rotatable shaft having an end inserted through the aperture in the housing; a nub on the inserted end of the shaft and in the slot in the piston, said nub being eccentrically positioned on the inserted end of the shaft so as to describe a circle with a diameter equal to the length of the slot in the piston when the shaft rotates; and means for rotating the shaft.

2. A pump for discharging varying quantities of liquid at a constant pressure, said pump comprising, in combination, a housing having a first end, a second end, a bore extending through it from its first end to its second end, and an inlet hole extending through it from the bore; a first cap closing the bore at the first end of the housing, said cap having a passage through it from the bore; a second cap closing the bore at the second end of the housing, said cap having a passage through it from the bore; a first sleeve slidably positioned in the bore between the first cap and the inlet hole and having a hole extending through it; a second sleeve slidably positioned within the bore between the sec-ond cap and the inlet hole and having a hole extending through it; a piston having an exterior surface, a first end slidably movable in the hole extending through the first sleeve, a second end slidably movable in the hole extending through the second sleeve, a first duct extending through it from its first end to its exterior surface between the first sleeve and the second sleeve, a second duct extending through it from its second end to its exterior surface between the first sleeve and second sleeve, and a length less than the distance between the first cap and the second cap; a plurality of springs extending between the first sleeve and the second sleeve and urging the first sleeve toward the first cap and the second sleeve toward the second cap with a pressure substantially equal to the constant pressure; a first suction valve positioned to permit the flow of liquid only from that end of the first duct at the first end of the piston; a second suction valve positioned to permit the flow of liquid only from that end of the second duct at the second end of the piston; a first check valve positioned in the first passage to permit the fiow of liquid only from the bore; a second check valve positioned in the second passage to permit the fiow of liquid only from the bore, and means for moving the first end of the piston toward and away from the first cap.

3. A pump for discharging liquid at a constant pressure, said pump comprising, in combination, a housing having an end, a bore extending into it from said end, and an inlet hole extending through it from the bore; a cap closing the bore at the end of the housing, said cap having a passage through it from the bore; means for storing said liquid when the pressure at said passage is at least equal to a predetermined pressure including a sleeve slidably positioned in the bore between the cap and the inlet hole and having a hole extending through it; a piston having an end slidably movable in the hole extending through the sleeve and a duct extending through it from said end into a portion of the bore more remote from the cap than the sleeve; a plurality of springs urging the sleeve toward the cap with a pressure substantially equal to the constant pressure; a suction valve positioned to permit the flow of liquid only from that end of the duct at the said end of the piston; a check valve positioned in the passage to permit the flow of liquid only from the bore; and means for moving the said end of the piston toward and away from the cap.

4. A pump for pumping a liquid, said pump comprising, in combination, a housing having an end and a bore extending into it from said end; a cap closing the bore at the end of the housing, said cap having a passage through it from the bore; means for storing liquid when the pressure at said passage is at least equal to a predetermined pressure including a sleeve slidably positioned in the bore and having a hole extending through it; a piston having an end slidably movable in the hole extending through the sleeve; a plurality of springs urging the sleeve toward the cap; means for introducing the liquid into the hole through the sleeve and between the cap and the said end of the piston; and means for moving the said end of the piston toward and away from the cap.

5. A pump for pumping a liquid at a constant pressure, said pump comprising, in combination, a housing having a hollow bore therethrough, said bore having an end; a

sleeve slidably movable in said bore toward and away from the end of said bore, said sleeve having a hole therethrough; a piston positioned within said bore and having an end slidably movable within the hole through said sleeve, said end of said piston and said sleeve forming a substantially liquid-tight barrier in said bore; a plurality of springs positioned in said bore and urging said sleeve against the end of said bore with a pressure substantially equal to said constant pressure; means for moving the end of said piston toward and away from the end of said bore; means for introducing liquid into said bore between the liquid-tight barrier and the end of said bore when the end of said piston is moved away from the end of said bore; and means for discharging liquid from said bore between the liquid-tight barrier and the end of said bore when the end of said piston moves toward the end of said bore.

6. A pump for pumping a liquid, said pump comprising, in combination, a housing defining a bore therein, said bore having an end; a sleeve slidably movable in said bore toward and away from the end of said bore, said sleeve having a hole therethrough; a piston positioned within said bore and having an end slidably movable within the hole through said sleeve, said end of said piston and said sleeve forming a substantially liquidtight barrier in said bore; a plurality of springs positioned in said bore and urging said sleeve against the end of said bore; means for moving the end of said piston toward and away from the end of said bore; means in said piston for introducing liquid into said bore between the liquid-tight barrier and the end of said bore when the end of said piston is moved away from the end of said bore; and means for discharging liquid from said bore between the liquid-tight barrier and the end of said bore when the end of said piston moves toward the end of said bore.

7. A pump for pumping a liquid, said pump comprising, in combination, a housing having a bore within it, said bore having an end; a sleeve slidably movable in said bore toward and away from the end of said bore, said sleeve having a hole therethrough; a piston positioned within the said bore and having an end slidably movable within the hole through said sleeve, said end of the piston and said sleeve forming a substantially liquid-tight barrier in said bore; means for slidably urging said sleeve against the end of said bore; means for moving the end of said piston toward and away from the end of said bore; means for introducing liquid through said piston into said bore between the liquid-tight barrier and the end of said bore when the end of said piston is moved away from the end of said bore; and means for discharging liquid from said bore between the liquid-tight barrier and the end of said bore when the end of said piston moves toward the end of said bore.

8. A pump for pumpim a liquid, said pump comprising, in combination, a housing having therein, said bore having an end; a sleeve slidably movable in said bore toward and away from the end of said bore, said sleeve having a hole therein; a piston positioned within said bore and having an end slidably movable within the hole through said sleeve, said end of said piston and said sleeve forming a substantially liquid-tight barrier in said bore; means for slidably urging said sleeve against the end of said bore; means for moving the end of said piston toward and away from the end of said bore; and means for introducing liquid into and releasing liquid from said bore between the end of said bore and the liquid-tight barrier.

9. A pump for pumping a liquid, said pump comprising, in combination, a housing having a bore therein, said bore having an end; a sleeve slidably movable in said bore against and away from the end of said bore, said sleeve having a hole therethrough; a piston positioned within said bore and having an end slidably movable within said hole through said sleeve, said end of said piston and said sleeve forming a substantially liquid-tight barrier in said bore; means for independently moving the sleeve and said piston toward the end of said bore; and means in said piston for introducing liquid into and releasing liquid from said bore between the liquid-tight barrier and the end of said bore.

10. A pump for pumping a liquid at a constant pressure, said pump including a housing having a bore therein, an end plate closing said bore, said end plate having an outlet port therein communicating with said bore, a piston having an end reciprocable within said bore toward and away from said end plate, means for introducing a liquid into said bore between said piston end and said end plate as said piston end moves away from said end plate, and means surrounding said piston end and engaging said end plate for storing said liquid when the pressure at said outlet port is at least equal to said constant pressure.

11. A pump for pumping liquid at a constant pressure, said pump including a housing having a bore therein, an end plate closing said here, said end plate having an outlet therein communicating with said bore, a piston having an end reciprocable within said bore toward and away from said end plate, means for introducing a liquid into said bore between the end of said piston and said end plate as said piston moves away from said end plate, means immediately adjacent the said end of said piston and engaging said end plate for storing said liquid when the pressure at said outlet port is at least equal to said constant pressure and for discharging the stored liquid through said outlet when the pressure at said outlet port is below said constant pressure.

References Cited by the Examiner UNITED STATES PATENTS 2,041,422 5/1936 LOrange 10337 2,956,501 10/1960 Norlin 103-37 3,016,837 1/1962 Dlugos 103-37 3,067,688 12/1962 Wirsching 10337 FOREIGN PATENTS 122,408 11/ 1944 Australia. 552,443 l/1923 France.

LAURENCE V. EFNER, Primary Examiner. 

1. A PUMP FOR DISCHARGING VARYING QUANTITIES OF LIQUID AT A CONSTANT PRESSURE, SAID PUMP COMPRISING, IN COMBINATION, A HOUSING HAVING A FIRST END, A SECOND END, A CYLINDRICAL BORE EXTENDING THROUGH IT FROM ITS FIRST END TO ITS SECOND END AND WITH A CENTERLINE, AND INLET HOLE EXTENDING THROUGH IT FROM THE BORE AND POSITIONED ALONG THE LENGTH OF THE BORE BETWEEN THE FIRST END AND THE SECOND END OF THE HOUSING, AND AN APERTURE EXTENDING THROUGH IT FROM THE BORE AND POSITIONED BETWEEN THE FIRST END AND THE SECOND END OF THE HOUSING; A FIRST CAP POSITIONED ACROSS AND CLOSING THE BORE AT THE FIRST END OF THE HOUSING, SAID CAP HAVING A FIRST PASSAGE EXTENDING THROUGH IT FROM THE BORE; A SECOND CAP POSITIONED ACROSS AND CLOSING THE BORE AT THE SECOND END OF THE HOUSING, SAID CAP HAVING A SECOND PASSAGE EXTENDING THROUGH IT FROM THE BORE; A FIRST SLEEVE SLIDABLY POSITIONED IN THE BORE BETWEEN THE FIRST CAP AND THE POSITION OF THE INLET HOLE ALONG THE LENGTH OF THE BORE AND HAVING A HOLE EXTENDING THROUGH IT WITH A CENTERLINE WHICH COINCIDES WITH THE CENTERLINE OF THE BORE; A SECOND SLEEVE SLIDABLY POSITIONED WITHIN THE BORE BETWEEN THE SECOND CAP AND THE POSITION OF THE INLET HOLE ALONG THE LENGTH OF THE BORE AND HAVING A HOLE EXTENDING THROUGH IT WITH A CENTERLINE WHICH COINCIDES WITH THE CENTERLINE OF THE BORE; A PISTON HAVING A CYLINDRICAL EXTERIOR SURFACE, A FIRST END SLIDABLY MOVABLE IN THE HOLE EXTENDING THROUGH THE FIRST SLEEVE, A SECOND END SLIDABLY MOVABLE IN THE HOLE EXTENDING THROUGH THE SECOND SLEEVE, A CENTERLINE COINCIDING WITH THE CENTERLINE OF THE BORE, A FIRST DUCT EXTENDING THROUGH IT FROM ITS FIRST END TO A PORTION OF ITS EXTERIOR SURFACE BETWEEN THE FIRST SLEEVE AND THE SECOND SLEEVE, A SECOND DUCT EXTENDING THROUGH IT FROM ITS SECOND END TO A PORTION OF ITS EXTERIOR SURFACE BETWEEN THE FIRST SLEEVE AND THE SECOND SLEEVE, A SLOT IN ITS EXTERIOR SURFACE WITH ITS LENGTH PERPENDICULAR TO THE CENTERLINE OF THE PISTON, AND A LENGTH LESS THAN THE DISTANCE BETWEEN THE FIRST CAP AND THE SECOND CAP; A PLURALITY OF SPRINGS EXTENDING BETWEEN THE FIRST SLEEVE AND THE SECOND SLEEVE, EQUIDISTANTLY POSITIONED AROUND THE PISTON, AND URGING THE FIRST SLEEVE TOWARD THE FIRST CAP AND THE SECOND SLEEVE TOWARD THE SECOND CAP WITH A FORCE SUBSTANTIALLY EQUAL TO THE CONSTANT PRESSURE; A FIRST SUCTION VALVE LOCATED AT THE FIRST END OF THE PISTON AND POSITIONED TO PERMIT THE FLOW OF LIQUID ONLY FROM THAT END OF THE FIRST DUCT AT THE FIRST END OF THE PISTON; A SECOND SUCTION VALVE LOCATED AT THE SECOND END OF THE PISTON AND POSITIONED TO PERMIT THE FLOW OF LIQUID ONLY FROM THAT END OF THE SECOND DUCT AT THE SECOND END OF THE PISTON; A FIRST CHECK VALVE LOCATED IN THE FIRST PASSAGE AND POSITIONED TO PERMIT THE FLOW OF LIQUID ONLY FROM THE BORE; A SECOND CHECK VALVE LOCATED IN THE SECOND PASSAGE AND POSITIONED TO PERMIT THE FLOW OF LIQUID ONLY FROM THE BORE; A ROTATABLE SHAFT HAVING AN END INSERTED THROUGH THE APERTURE IN THE HOUSING; A NUB ON THE INSERTED END OF THE SHAFT AND IN THE SLOT IN THE PISTON, SAID NUB BEING ECCENTRICALLY POSITIONED ON THE INSERTED END OF THE SHAFT SO AS TO DESCRIBE A CIRCLE WITH A DIAMETER EQUAL TO THE LENGTH OF THE SLOT IN THE PISTON WHEN THE SHAFT ROTATES; AND MEANS FOR ROTATING THE SHAFT. 